[time-nuts] Aerial coax downlead placement

[Bob kb8tq]

Hi

The purpose of coax is to shield the signal. The outer portion of the cable 
acts to protect the inner part from stray signals in the environment. In a normal 
system, the outer braid is connected to ground. It is no different than a lot of audio
cabling in that respect. 

Energy flow is indeed inside the cable if things are set up and operating correctly.
If it was on the outside, the shield would not be doing its job of protecting things. 
This is only true to the extent that skin depth will allow it to happen. 

Why does this matter? With something like a 1 pps timing pule, some portion of the 
energy *will* be low enough in frequency to make it past the skin depth / thickness 
of any practical cable. The components that create the fast rising edge will be contained, 
but the low frequency stuff will not be. Fortunately we rarely use cables that are a 
significant fraction of a wavelength at 1 Hz :)

Bob

> On Jul 5, 2019, at 1:47 PM, Peter Vince <petervince1952 at gmail.com> wrote:
> 
> On Fri, 5 Jul 2019 at 18:01, WigglePig <wigglepig at gmail.com> wrote:
> 
>> If there are currents in the braid to upset then your antenna system is
>> not working as you might believe.
>> 
> 
> I was working on the simplistic assumption that for a current to flow,
> there must be a complete circuit, so the current flows down the centre
> conductor, and must come back up the braid.  But I gather that unlike DC,
> RF is "black magic", and only flows on the inside of the braid - if all the
> impedances match.
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